Transmission lines are important elements in microwave circuit applications. Transmission lines provide the interconnection between active and passive devices of microwave circuits, and are utilized as impedance matching elements as well. A strip-line is a type of transmission line widely utilized in monolithic microwave integrated circuit (MMIC) applications.
Strip-lines have a number of advantages when utilized in MMIC applications. First of all, since microstrip-lines are formed of conductive planes disposed on substrates, they are readily adaptable to the manufacturing process of the integrated circuits. Accordingly, strip-lines may be integrated on a same substrate with commonly used integrated circuits such as complementary metal-oxide-semiconductor (CMOS) circuits.
In a conventional grounded coplanar wave-guide (GCPW), which is a type of strip-line, a signal line is shielded from the underlying substrate by a grounded metal plane that is inserted between the substrate and the signal line. It is realized that even if the signal line is formed in the top metal layer of the respective chip, the distance between the signal line and the grounded metal plane is still very small. Accordingly, the capacitance between the signal line and the grounded metal plane is small. The characteristic impedance of the GCPW, which characteristic impedance is proportional to a square root of a quotient of an inductance of the GCPW to the capacitance, is also small. However, the GCPW needs to have a characteristic impedance matching the characteristic impedance of the connecting devices. This requires that either the width of the signal line is reduced, or the spacing between the signal line and the grounded metal plane is increased. Since the spacing is limited by the distance between the top metal layer and the substrate, the width of the metal line has to be very small, which not only incurs process difficulty, but also results in the resistance of the signal line to be adversely increased.